To engage an automatic transmission into a specific gear mode, i.e., park (P), reverse (R), neutral (N), drive (D or 4), and low drives (1, 2 and 3), a manual valve must be moved into proper position so that hydraulic pressure can be directed to appropriate transmission components. Likewise, a parking rod must be positioned for proper engagement when in (P). The manual valve and parking rod are linked to the transmission selector lever (located in the passenger compartment) via a detent lever, within the transmission case. Conventional mode switches, which are designed to provide an electrical signal based on the gear selected, are connected to this linkage and are located outside the transmission case.
With conventional mode switches, an electrical signal is generated by sliding contacts. As a linkage rotates, electrical contacts slide along a uniquely designed pad to produce electrical signals. Such electrical signals can be used in transmission control algorithms and with electronic gear-selection displays.
Some of the more prevalent problems with this type of external, mechanical switch are: (1) positional adjustment is required during installation; (2) poor accuracy results from the tolerance stack-up of the many components involved (i.e., the output of the switch may indicate a gear mode that has not been selected); (3) sealing from the environment is required; and (4) the sliding contacts of these mechanical switches are prone to corrosion and wear, which results in poor electrical contact.
Magnetic field sensors have been proposed to replace conventional mode switches to indicate a selected gear of an automatic transmission. Such sensors would detect binary control signals generated when a code slide with a pattern or protuberances is moved across a magnetic field. However, these proposed magnetic field devices relate to sensors disposed outside the transmission housing and indicate the position of a shift lever, not the gear mode of the transmission itself. For those reasons, such devices solve few of the listed problems associated with external mechanical switches.
The foregoing illustrates limitations known to exist in present sensors to detect a selected gear of an automatic transmission. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.